Proper parsing of empty dnsHosts string

When DNS hosts are empty, plugin sends DNS override to docker container
("Dns":[""]).  With this patch, correctly declines to override dns when
the cloud configuration does not ask for it.
pull the image if we do not find it
wait until the pulling of the image is complete before moving on
Merge pull request #45 from mgius/fix_empty_dns_hosts

Proper parsing of empty dnsHosts string
Merge pull request #49 from DustinChaloupka/pull-image-if-none

Pull image if none
Added 'volumes-from' functionality